1. You can disclose in your profile.
This is a direct way to tell other people that you are trans. When find dating partner on ordinary dating sites, there are no gender options for transgender people, so disclose who you are in your profile.
2. Tell them directly
If you don't know how to express yourself in your profile, the best way is to tell your partner who you are when you meet each other for the first time. This is also a way to show your respect to your partner.